The present application is related to optical surfaces formed on a substrate e.g. wafer, particularly to optical surfaces formed on a substrate in a fully or partially recessed state.
Replication has become an increasingly popular technique of forming lenses, especially in bulk. In this method, a flat substrate is provided, typically a wafer made of glass (although other substrates may be used) upon which is placed a replication material or other formable material. The replication material is then shaped by a variety of curing methods; a common approach is that of stamping the replication material with a master and curing the material to form the desired optical surfaces. Upon curing the replication material, the master can be removed, leaving behind the formed lens shapes on the wafer surface. One or both sides of the substrate can be populated with replicated lenses, and in some cases the lenses on both sides may be formed simultaneously. Thereafter, the substrate can be diced into individual lenses, although in some cases, the wafer is left intact to be joined to one or more other lens or spacer wafers to form multiple layers of lenses that are then singulated. Wafer-level fabrication of lenses is often preferred since the cost per lens is quite low compared to many other methods.
One other driving factor in the market is the tendency of lenses and consequently cameras in which they are placed to become smaller. Replication and wafer-level processing have enabled shrinking of lenses and lens stacks in both width and in thickness. Even so, the flat substrate must be thick enough to have sufficient strength and stability, and having one or two extra replicated layers with the formed lens layer adds to the overall height of each lens wafer. With increasing complexity of applications including higher density image sensors in cameras, there is greater use of multiple optical surfaces in a lens stack. Added layers compound the problem of lens heights as the thickness of the resulting lens designs may be higher than what is desired. Although this is true of both convex and concave lenses, convex lenses often pose an additional challenge. When the tops of the convex lenses are the highest points on a lens wafer, this can cause difficulties with any processing step that requires the wafer to touch another surface. Unprotected convex lenses on the surface are generally more exposed to harm from incidental damage as well.